While iterating over each item in the List<string> retrieved from an AJAX GET request in JavaScript

Trying to iterate through a list of strings and display them on the page, but facing an error as described in the title...

"Uncaught TypeError: response.forEach is not a function"

I've looked into for loops in JavaScript, but they seem to work with arrays. As a JavaScript novice, I'm wondering if there is an equivalent .ToArray() method in JavaScript that I can use? I have created a simple application to demonstrate my goal.

Note: The return type of the controller method "GetStringList" cannot be changed. In the larger application, the method must return a list of strings.


Controller Method (C#)

    public List<string> GetStringList(){
        List<string> listOfStrings = new List<string>();

        SqlConnection sqlConn = new SqlConnection(connection_String);

        sqlConn.Open();

        string sqlStr = "SELECT * FROM dbo.[SampleOrders] WHERE Active = 1";
        SqlCommand sqlCmd = new SqlCommand(sqlStr, sqlConn);
        SqlDataReader sqlDR = sqlCmd.ExecuteReader();

        if (sqlDR.HasRows) {
            while (sqlDR.Read()) {
                listOfStrings.Add((string)sqlDR["SalesOrder"]);
            }
        }

        sqlDR.Close();
        sqlCmd.Dispose();
        sqlConn.Close();

        return listOfStrings;
    }

View / JavaScript Function

    <script>
        $(document).ready(function () {
            $('#demoBtn').click(function () {
                $.ajax({
                    type: 'GET',
                    url: '/Home/GetStringList',
                    success: function (response) {
                        response.forEach(function (item) {
                            alert(item);
                        });
                    }
                });
            });
        });
    </script>

Any clarifications needed, please feel free to ask - appreciate your help!

Answer №1

.forEach() is a built-in method for arrays. The variable response stores the string returned by an XMLHttpRequest. In JavaScript, there is no specific type like List<String>, so you receive a single string.

To work with this data, consider converting the string into an array using

var myArray = JSON.parse(response)
. Once parsed, you can then use the .forEach() method on myArray.

It might be helpful to log the contents of response to understand its exact format and structure better.

Answer №2

It is not advisable to simply return listOfStrings as it will only display something like

System.Collections.Generic.List 1[System.String]

To address this, consider modifying your code as follows:

return string.Join(",",listOfStrings);

If you want to access the list using Javascript, I recommend returning it as JSON, like this:

using Newtonsoft.Json;
...
return JsonConvert.SerializeObject(listOfStrings);

This will result in a string format similar to:

["string1","string2","string3"]

Next, update your jQuery to expect JSON:

$.ajax({
  type: 'GET',
  dataType: 'json', //This ensures that the response is parsed correctly for use in Javascript
  url: '/Home/GetStringList',
  success: function (response) {
    response.forEach(function (item) {
      alert(item);
    });
  }
});

If you are unable to modify the return statement, there are two alternatives:

  • Include an optional parameter to indicate when JSON is needed
public List<string> GetStringList(bool json = False){
  ...
  if(json){
    return JsonConvert.SerializeObject(listOfStrings);
  }
  return listOfStrings;
}
  • Create a separate method to encapsulate the JSON return:
public string GetStringListAsJSON(){
  return JsonConvert.SerializeObject(GetStringList());
}

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